Inductance and capacitance reactance measuring instrument having minimal inclusion of stray reactances

ABSTRACT

An impedance-measuring electrical circuit modulates a high frequency current alternately with the unknown impedance being measured and with a reference impedance. A detector synchronized with the alternation of the modulation detects the resultant modulated current to produce a measure of the resistive and reactive components of the unknown impedance element relative to the corresponding components of the known reference impedance element. The modulating portion of the circuit is constructed with minimal stray reactances that effect the modulated output current, but the other portions of the circuit are essentially free of this restriction.

United States Patent 1 Hendriks Feb. 27, 1973 INDUCTANCE AND CAPACITANCEREACTANCE MEASURING INSTRUMENT HAVING MINIMAL INCLUSION OF STRAYREACTANCES 3,584,295 6/l97l Bayer .324/57 Primary Examiner-Stanley T.Krawczcwicz Attorney-Kenway, Jenney & Hildreth 5 7 ABSTRACT Animpedance-measuring electrical circuit modulates a high frequencycurrent alternately with the unknown impedance being measured and with areference impedance. A detector synchronized with the alternation of themodulation detects the resultant modulated current to produce a measureof the resistive and reactive components of the unknown impedanceelement relative to the corresponding components of the known referenceimpedance element. The modulating portion of the circuit is constructedwith minimal stray reactances that effect the modulated output current,but the other portions of the circuit are essentially free of thisrestriction.

13 Claims, 5 Drawing Figures J 'P'HKSE DETECTOR DELAY l 82 W l SAMPLE aHOLD 74 l l 4 l VOLTAGE I i f CONTROLLED I l OSCILLATOR L- h 66 l l IAVERAGE SWITCH I DETECTOR 70 l CONTROLLER| 1, IT 62 SAMPLE SAMPLE 78 T I26 T a HOLD 8 HOLD 1 Z X 2 :3 i

40 OUTPUT I 42 coNvERTER@ OUTPUT OONVERTER PATENTEDFEBZHEUS SHEET 10F 2INVENTOR JOZEF H. H EN DRI KS ATTORNEYS INDUCTANCE AND CAPACITANCEREACTANCE MEASURING INSTRUMENT HAVING MINIMAL INCLUSION OF STRAYREACTANCES BACKGROUND This invention relates to the measurement ofelectrical reactances with minimal effect from stray reactances in themeasuring equipment. More particularly, the invention provides anelectrical reactancemeasuring instrument that provides an improvedreduction in the stray reactances included in the measure. The inventionfurther provides an electrical measuring instrument having improvedcapability for measuring accurately the reactance of miniature circuitelements.

Instruments for measuring reactances at radio frequencies, whichtypically are of the bridge form, are constructed according to one priorart technique with all the radio frequency portions having utmostcompactness to minimize the length of the interconnections and otherwiseminimize stray reactances-Instruments constructed in this mannergenerally are costly, because the measuring instrument has relativelyextensive radio frequency circuits and considerable effort therefore isrequired to construct them all with uniformly low stray reactances.

Another prior art technique for r. f. reactance measurement with minimalerror from stray reactances, which can be practiced with bridge-typecircuits, requires the delivery of a known radio frequency current tothe instrument terminals connected to the unknown circuit element, andthe measurement of the voltage across these terminals. The circuitry forproviding this operation, although seemingly simple in concept, iscomplex and hence also is costly.

Accordingly, it is an object of this invention to provide an improvedcircuit for measuring electrical reactances, particularly at radio andlike high frequencies.

Another object of the invention is to provide a reactance measuringcircuit having minimal dependence on stray reactances in the circuit, asdistinguished from the reactance of the unknown element being measuredand whatever reference reactances are purposely provided in the circuit.

A further object of the invention is to provide a reactance-measuringcircuit in which only a minimal portion needs to be constructed with lowreactance in order to provide an accurate measure. That is, it is anobject to provide a reactance-measuring circuit in which only the strayreactances in a small part of the circuit effect the measurement.

Another object of the invention is to provide an improved instrument formeasuring reactances in integrated and like miniature circuits, and inparticular to provide such an instrument operating with minimal errordue to stray reactances.

It is also an object of the invention to provide measuring circuits ofthe above character that can measure both inductive and capacitivereactances, and that can use different kinds of reference circuitelements.

A further object of the invention is to provide measuring equipment ofthe above character that can be constructed at a relatively low cost.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

GENERAL DESCRIPTION One measuring circuit embodying the invention andfor use with an unknown capacitive circuit element has, considered ingeneral, a modulator and a demodulator. The modulator modulates thephase and amplitude of a radio or like frequency current in alternatetime periods with the admittance of the circuit element being measuredand then with the known admittance of a reference circuit element. Theresultant modulated current contains all the information required toidentify the unknown admittance relative to the known referenceadmittance. The demodulator receives this current and, operating insynchronism with the alternation of the modulator, produces an outputsignal measuring the capacitance of the unknown circuit element relativeto the reference element. Where desired, the demodulator furtherprocesses the modulated current to produce also a measure of theconductance of the unknown circuit element relative to the referenceelement.

With this arrangement, the demodulator can readily be constructed foraccurate operation, so that essentially only stray reactances in themodulator detract from accurate measurement of the unknown circuitelement. Moreover, the modulator consists of relatively few circuitcomponents, and only certain limited stray reactances in the modulatordetract from the measuring accuracy. Consequently, the modulator can beconstructed at comparatively low cost with the particular strayreactances that detract from the measuring accuracy held to a minimum.As a result, the complete measuring circuit can be constructed atrelatively low cost and yet provide reactance measurements with a highdegree of accuracy, i.e., with minimal error due to stray reactances.

Further, the modulator portion of the measuring equipment can beconstructed compactly, e.g., with densely-assembled miniaturecomponents. The miniature modulator can then be located directly at, orat least very close to, the unknown circuit element being measured, tominimize the length of the leads connecting the circuit element undertest to the modulator. For example, an instrument for measuringreactances in miniature integrated circuits can incorporate themodulator portion in the probe that contacts the circuit structure beingtested. Thedemodulator can be located at a convenient location removedfrom the modulator, and connected to it by cable or even by a wirelessradio link.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts exemplified in theconstructions hereinafter set forth, and the scope of the invention isindicated in the claims.

BRIEF DESCRIPTION OF DRAWINGS form, of a capacitance-measuringinstrument embodying the invention;

FIG. 2 is a plot of current as a function of time illustrating theoperation of the modulator portion of the FIG. 1 instrument;

FIG. 3 is a vector diagram pertinent to the operation of the measuringcircuit of FIG. 1;

FIG. 4 is a schematic representation, partly in block form, of anothercapacitance-measuring instrument embodying the invention; and

FIG. 5 is a vector diagram pertinent to the operation of the measuringcircuit of FIG. 4.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS FIG. 1 shows acapacitance-measuring instrument having a modulator section indicatedgenerally at in which a radio frequency source 12 applies current,schematically by way of switches 14 and 18, to a terminal 16 alternatelythrough the unknown admittance of a circuit element 20'being measured,and through the known admittance of a reference circuit element 24. Aswitch controller 26 operates the switches 14 and 18 asynchronously,opening one when it closes the other, at periodic intervals many timeslonger than the period of the source 12. Accordingly, the currentleaving the demodulator 10 at the terminal 16 has a waveform of the typeshown in FIG. 2. Curve 28 forming the top half of the waveform envelopecorresponds to the switching operation of switches 14 and 18.

Thus, during the time interval t switch 14 is closed and switch 18 isopen, and the modulator output current is the the current i,, shown withwaveform 30, through the circuit element 20. In the other time intervalt when switch 14 is open and switch 18 is closed, the condition shown inFIG. 1, the modulator output current is the current 1, through thereference circuit element 24. This current has the waveform 32, FIG. 2.The two current waveforms 30 and 32 have the same frequency, identicalto the frequency of the source 12, but have different magnitudes andhave different relative phases, as indicated by the discontinuitiesbetween the two curves 30 and 32 at their junctures.

With further reference to FIG. 1, the measuring circuit has ademodulator or detection portion 34 that receives the modulated currentfrom the modulator terminal 16, and performs two demodulations in analternate sequence synchronized with the operation of the switchcontroller 26 to produce at a terminal 36 a voltage v proportional tothe susceptance of the circuit element 20 and at a terminal 38 a voltagev propor tional to the conductance of the reference element 24. Theillustrated demodulator further produces, at terminal 40, a voltage vproportional to the conductance of the unknown element 20. An outputconverter 42 receives the voltages v and v and in response produces anoutput indication identifying the capacitance of the circuit element 20relative to the reference element 24. A further converter 44 indicatesthe value of the conductance of the unknown element 20, also relative tothe reference element 24.

The details of the illustrated demodulator are discussed below afterfurther consideration of the modulator 10.

The source 12, operating at the frequency at which the circuit element20 is to be measured, is connected between the moving contacts of theswitches 14 and 18, which together operate as a double-pole,double-throw switch. The switch fixed contacts are connected to theunknown circuit element 20, known element 24, and

ground or other return conductor as shown. With this arrangement, whenswitches 14 and 18 are reversed from the position shown the sourcevoltage is applied effectively as voltage e across the circuit element20 being measured, and the modulator produces the funknown current iWith the switches in their other positions as shown, the source voltageis applied with opposite phase, as voltage e,, to the reference circuitelement 24 and the modulator produces the reference current i The switchcontroller 26 operates the switches cyclically, opening switch 14 andclosing switch 18 for a half-cycle time 1 and reversing the switchesduring the other half-cycle time t,. FIG. 1 shows the measuring circuitwith these switches and others discussed below in the positions theyhave during each t interval.

The reference circuit element 24 of the illustrated modulator is aresistor of known value, expressed as a conductance G The referenceelement preferably has minimal reactance, i.e., it is purely resistive.The unknown circuit element 20 is illustrated as a capacitor 20a havinga value C x and shunted by a resistor 20b of value G The circuit element20 is connected to the modulator at terminals 46, 46.

With this illustrated modulator construction, the voltages e and e, canbe represented by vectors 48 and 50 respectively, as shown in FIG. 3.Further, vector 52 represents the reference current i and is in phasewith the voltage vector 50 because the reference element is purelyresistive. (The demodulator input amplifier 56 is assumed to have aninput impedance that is purely resistive and which preferably is smallrelative to the impedance being measured.) The length of vector 52 isproportional to the conductance value G of the element 24.

FIG. 3 also shows a vector 54 representing the unknown current i,. Thisvector leads the voltage vector 48 by less the angle 111 due to theresistor 20b. That is, the vector 54 is the resultant of a vectorcomponent 54a that represents the current in the unknown capacitor 20aand of a vector component 54b that represents the current in the unknownresistor 20b. The length of vector component 54a is proportional to thecapacitance C X and the length of the component 54b is proportional tothe conductance G The current output from the demodulator is the vector54 current i during each time interval t and is the vector 52 current iduring each time interval as shown in FIG. 2. The FIG. 3 vectorrepresentation of these two currents shows that they contain all of theinformation needed to identify the capacitance C and the conductance G Xof the circuit element 20 relative to the known admittance, i.e.,conductance G of the reference element 24.

The demodulator 34 extracts this information from the current itreceives from the modulator 10 and displays the resultant values withsuitable indicators on the converters 42 and 44.

With further reference to FIGS. 1, 2 and 3, the demodulator 34 amplifiesthe current from the modulator terminal 16 with an amplifier 56 havingan automatic gain control (AGC) input terminal 56a. During the timeperiods when demodulator switch 18 is closed, a switch 58, illustratedas a single-pole, single-throw switch and operated by the switchcontroller 26 in synchronism with switches 18 and 14, applies theamplifieroutput signal to an average detector .60. The detector, whichthus receives only the portion of the amplifier output signal thatcorresponds to the reference current i applies a resultant detecteddirect voltage to a sample and hold unit 62. The signal output from theunit 62 is the voltage v noted above; it is proportional to themagnitude of the FIG. 3 vector 52, i.e., to the known conductance G ofthe circuit element 24. This voltage is applied from the output terminal38 to the amplifier 56 AGC input terminal 56a, in addition to beingapplied to each output converter 42 and 44.

The reason the voltage v is applied to the amplifier 56 AGC inputterminal is to maintain the gain of the amplifier during each t intervalthe same as it was in the preceding t interval, i.e., so that theamplifier has the same gain for the unknown current i as it has for thereference current The amplifier 56 output signal also is applied to oneinput of a phase detector 64. During the time intervals t,, a switch 66,schematically a single-pole, doublethrow switch operated with theswitches 14, 18 and 58, applies the phase detector 64 output signal to asample and hold network formed by a series resistor 66 and a shuntcapacitor 70. An operational amplifier 72 receives the voltage acrossthe capacitor 70 and drives a voltage-controlled oscillator 74 inresponse to it. The alternating signal output from the oscillator isapplied to the other input of phase detector 64.

This arrangement of the elements 64, 66, 68, 70, 72 and 74 forms asynchronous detector of the so-called phase-lock type and accordinglythe amplifier 72 drives the oscillator 74 to produce a radio frequencysignal identical in frequency to, and shifted 90 in phase from, thesignal the phase detector receives from the amplifier 56. This isbecause with these like-frequency and quadrature-phased signals input tothe phase detector 64, its output signal has null value and hence theamplifier 72 receives a null input signal. Accordingly, during the timeintervals t when the demodulator 34 receives the reference current i theelements 64-74 operate to develop the quadrature reference signal outputfrom the oscillator 74. The sample and hold circuit provided by resistor68 and capacitor 70 maintains the oscillator 74 in this operation duringthe alternate time intervals t when the switch 66 is removed from theposition shown.

During the time intervals t,, when switch 14 is closed, switches 18 and58 are open, and switch 66 is in the position shown with dashed lines,the phase detector 64 continues to receive the quadrature referencesignal from the voltage-controlled oscillator 74, but now receives fromthe amplifier 56 a signal corresponding to the unknown current i,. Inresponse to these signals, which have the relative phases of the vector54 and vector 48, the phase detector produces a direct voltage that isproportional essentially only to the magnitude of the FIG. 3 vectorcomponent 54a, and hence to the value of the unknown capacitance C Theswitch 66 applies this direct voltage to a sample and hold network 76,the output of which is the terminal 36 at which the demodulator developsthe voltage v, corresponding to the susceptance of the circuit element20 and hence to the value C x of capacitor 20a.

To detect the component of the i, current responsive to the conductancecomponent of the circuit element 20, the demodulator 34 has a 90 phaseshifter 78 that shifts by 90 the phase of the quadrature referencesignal from the oscillator 74 and applies it to one input of a furtherphase detector 80. The other input to the phase detector 80 is thesignal output from amplifier 56. A switch 82, operated in synchronismwith the switch 14 so as to be closed only during the t, intervals,applies the phase detector 80 output voltage to a sample and holdnetwork 84, the output voltage from which is the v voltage proportionalto the conductance G X of the circuit element 20, i.e., to vectorcomponent 54b.

With further reference to FIG. 1, the demodulator 34 is constructed withthe signal path from the amplifier 56 to the phase detector 80 havingthe same electrical length and hence the same phase delay as the signalpath from the amplifier 56 to the phasedetector 64. Further, the twophase detectors have identical internal delays to the r. f. signals theyreceive. Also, the signal path from oscillator 74 to detector 80,including delay unit 78, is longer by a quarter wavelength, i.e., thanthe path from the oscillator to detector 64. These are the onlyrequirements on the demodulator for accurate measuring operation. Thisis because the demodulator 34 processes both the reference current i andthe unknown current i with the amplifier 56, and applies both currentsto the phase detectors 64 and 80. Thus both modulated currents propagatein the demodulator on the same signal paths. Hence, with the foregoingconstruction of the r. f. paths in the demodulator, stray reactances inthe r. f. portion of the demodulator, i.e., between the amplifier 56input and into the two phase detectors and from the oscillator 74 to thetwo detectors, will effect both currents identically and have no neteffect on the accuracy of the demodulator operation.

1n the modulator 10, however, certain stray reactances will effect thetwo currents i, and i differently, and the modulator accordingly isconstructed with known techniques to minimize them. Specifically, themodulator 10 develops the voltages e, and-e with equal magnitudes andopposite relative phases, as indicated by the opposed FIG. 3 vectors 48and 50. Stray reactances, i.e., reactances other than of the circuitelement 20 being measured, in the path of current i, from the fixedcontact of switch 14 to terminal 16 will offset the relative phaseand/or magnitude of this current. Hence this path is constructed withessentially reactance-free components, particularly with components freeof series inductance. Shunt capacitances, i.e., to ground, in this pathare of lesser significance unless the input impedance of the demodulatoramplifier 56 is not made small relative to the impedance of the unknownelement 20 and of the reference element 24; hence a low amplifier inputimpedance is provided. Further, modulator capacitances in shunt with thecurrent i, can be cancelled by a neutralizing capacitor equal to theseshunt capacitances and driven in phase opposition to e e.g., with thevoltage e Similarly, the modulator 10 develops the current i, in a pathessentially free of stray reactances.

Thus, the modulator 10 is constructed with such essentiallyreactance-free components and interconnections that the phase andmagnitude of the unknown current i reflect only the real and imaginarycomponents of the element 20 impedance being measured, and the referencecurrent i has a known phase relative to the source voltages e, and e andhas a magnitude dependent only on the known conductance G FIG. 4 showsanother impedancemeasuring circuit embodying the invention and generallysimilar to the circuit of FIG. 1 except that-the former has a delay lineand a multiplier-type demodulator, rather than the phase lock-typedemodulator 34 of FIG. 1. More particularly, the FIG. 4 measuringcircuit has a modulator portion 90 that produces current with a waveformsimilar to that shown in FIG. 2, and has a demodulator portion 92 thatextracts the desired impedance-measuring signals from this current.

The modulator 90 is basically identical to the modulator of FIG. 1except that it employs a capacitive reference circuit element. It alsoillustrates one construction for the FIG. 1 switches 14 and 18 andswitch controller 26, and it employs transformer coupling between themodulator and the demodulator. In particular, the modulator 90 has aradio frequency oscillator or other source 94 exciting the primarywinding of a transformer 96 that has two secondary windings 96a and 96bconnected in series at a grounded tap 960. The secondary windings canhave different numbers of turns but for simplicity are illustrated ashaving a unity turns ratio. Further, the two secondary windings arearranged as indicated with the conventional dot notation, which meansthat when the upper end of the transformer primary winding is positive,the uppermost end of secondary winding 96a is positive and thebottommost end of winding 96b is negative. Consequently the voltagesdeveloped across the two secondary windings are out of phase asdesignated on the drawing with the voltages e, and e,'.

A circuit element 98 to be measured, and illustrated as including acapacitor 98a of capacitance C and shunted by a resistor 98b ofconductance G is connected between terminals 100, 100 in one loop of themodulator 90 to modulate a current i, in response to the voltage e,developed across the transformer secondary winding 96a. Similarly, areference circuit element 102 in the form of a capacitor with knowncapacitance C is connected in the other loop of the modulator 90 to drawa current i in response to the r. f. voltage e developed across thetransformer winding 96b. The primary winding 104a of a couplingtransformer 104 is connected between a terminal 106 interconnecting thetwo modulator loops and the interconnection 96c of the two secondarywindings 96a and 96b. The output signal from the modulator 90 is takenfrom the coupling transformer secondary winding 1114b.

With this arrangement, the coupling transformer primary winding 104areceives one of the currents i, or i, depending on which loop of themodulator is switched in circuit with the transformer 96 to receivecurrent from it. To effect this switching, the modulator 90 has a diode108 in series between the circuit element 98 and the winding 96a andpoled to conduct forward current from the circuit element to thewinding; and has a like diode 110 in series between, and poled toconduct current from, the winding 96b and the reference circuit element102. Further, a diode 112 is connected to conduct forward current fromthe transformer tap 960 to the anode of diode 108, and a further diode 114 is connected to conduct forward current from the cathode of diode tothe tap 960. Current limiting resistors 116 and 118 connect one side ofa multivibrator 120 respectively to the anode of diode 118 and thecathode of diode 110; the other side of the multivibrator is connectedto the ground return conductor.

When the multivibrator output voltage, which is a periodic square wavethat switches symmetrically about ground from a positive voltage to anegative voltage, applies a positive voltage to resistors 116 and 118,diodes 108 and 1 14 are forward biased, and diodes 112 and 1 10 arereversed biased. The reversed-biased diode l 10 blocks the sourcevoltage across winding 96b from the reference circuit element 102 andinstead the forward-biased diode 114 couples on side of this element toground to constrain the i current to zero at this time. However, theforward-biased diode 108 connects the unknown circuit element 98 andtransformer winding 104a in series with the winding 96a and themodulator develops the current i,. When the multivibrator 120 switchesso that it applies a negative potential to resistors 116 and 118, theconditions of the switching diodes are reversed, with the result thatthe coupling transformer winding 104a and the reference circuit element102 are in series with the secondary winding 86b and the modulator 90applies the current to the winding 104a; diode 112 now constrains thecurrent i, to zero. Thus, in successive time intervals determined by theswitching of the multivibrator 120, the modulator 90 develops a currentin the coupling transformer 104b that is alternately responsive to thecurrent i, and to the current i,. This operation of the FIG. 4 modulator90 is thus identical to the operation of the FIG. 1 modulator 10.

FIG. 5 shows in vector form the relative phases of the currents i and iVector 122 represents the current i, dependent on the reference circuitelement 102 and vector 124 represents the current i,. Due to theout-ofphase arrangement of the secondary windings 96a and 96b, the twovectors would be 180 out of phase if the unknown circuit element 98 werea pure capacitance. However, the presence of conductance G reduces theangle between the two vectors by the angle ill as indicated.Consequently, the vector 124 component 124a, which is 180 out of phasewith the vector 122, is a measure of the capacitance C of the circuitelement 98 and the quadrature component l24b is a measure of theconductance G of this circuit element.

As in the FIG. 1 demodulator 34, the demodulator 92 amplifies themodulated output current with an amplifier 126 receiving an automaticgain control (AGC) voltage responsive only to the reference current i,.FIG. 4 illustrates that this AGC voltage is developed with an averagedetector 128 and a d. c. amplifier 142. A switch 140, illustratedschematically as a single-pole, double-throw switch alternately appliesto the input of detector 128 the output signal from amplifier 126 andthe signal output from a delay line 152. The switch is operated insynchronism with multivibrator 120 so that both signals it applies tothe detector 128 are responsive to the reference current 1', input toamplifier 126. More specifically, the switch 140, typically constructedof semiconductor elements as are the modulator switches and controlledby the modulator multivibrator 120, switches from the position shown tothe other position indicated with a dashed line during each timeinterval when the modulator 90 produces the unknown current i,'.

The output voltage from the amplifier 142 is the automatic gain controlvoltage for the amplifier 126. It also is the demodulator output voltagev that represents the value of the reference capacitance C The remainderof the demodulator 92 is a synchronous detector constructed with a pairof linear four-quadrant multipliers 148 and 150, a delay line 152, and a90 phase shifter 154 having two out-ofphase output terminals which aswitch 156 selectively connects one at a time to the multiplier 150 insynchronism with the operation of multivibrator 120 and hence withswitch 140. The delay line 152 has a delay equal to one-half the periodof the multivibrator 120 operation. Thus, the delay is equal to thelength of each time interval t and t indicated in FIG. 2 or, in otherwords, is equal to the interval during which the modulator produceseither current i, or i When the demodulator switches 140 and 156 are inthe positions shown, corresponding to the time intervals when themodulator produces the reference current i the multiplier 148 receivesfrom the amplifier 126 a signal responsive to the reference current i,and hence to the FIG. 5 vector 122. From the delay line 152, themultiplier 148 receives a signal that is responsive to the output fromamplifier 126 during the previous half cycle of multivibrator 120operation; this signal is the amplifier 126 output signal responsive tothe unknown current i, and which corresponds to the FIG. 5 vector 124.

In response to these input signals, the multiplier 148 produces anoutput voltage corresponding to the projection of vector 124 on the axisof vector 122. Hence the multiplier produces an output voltagerepresenting the vector component 124a, which is the measure of thecapacitance C of the unknown circuit element 98.

When the multivibrator output voltage switches, the input signals to themultiplier 148 are identical but reversed, i.e., from the amplifier 126the multiplier receives a signal responsive to the unknown current i,and from the delay line 152 it receives a signal responsive to thereference current i produced during the immediately preceding half-cycleof the multivibrator 120 operation. Accordingly, the multiplier outputvoltage is the same, i.e., is responsive to the vector component 124a.Thus, the output voltage v from the multiplier 148 is a constantrepresentation or measure of the capacitance C;' of the unknown circuitelement 98.

The other multiplier 150 of the demodulator 92 operates in a similarmanner but receives the amplifier 126 output signal with a 90 phaseshift and hence produces an output voltage, v that is a constant measureof the conductance G of the circuit element 98. That is, when themultivibrator 120 operates the modulator 90 to produce the referencecurrent i,, during which time the demodulator switches 140 and 156 arein the positions shown, the multiplier 150 receives, by way of the phaseshifter output terminal 154a and switch 156, the amplifier 126 outputsignal shifted in phase by -90, and it receives by way of delay line 152the amplifier output signal at the immediately preceding time interval.The former signal represents the reference current i vector 122 delayedin phase by and hence represents the FIG. 5 vector 160. The lattersignal represents the unknown current i vector 124. In response to thesesignals, the multiplier 150 produces the v output voltage according tothe projection of bector 124 aligned with vector 160, and thisprojection is identical to the vector component 124k. Accordingly, themultiplier 150 output voltage during this time interval is a measure ofthe conductance G X of the unknown circuit element 98.

When the multivibrator switches so that the modulator 90 produces theunknown current i,', and the switches and 156 are switched from thesolidline positions shown in FIG. 4, multiplier receives via delay line152 the amplifier 126 output signal responsive to vector 122. From thephase shifter 154 terminal 154b, and by way of switch 156, themultiplier in addition receives the amplifier 126 output signalresponsive to the unknown current vector 124 advanced in phase by 90and, accordingly, corresponding to the FIG. 5 vector 158. In response tothese two signals, the multiplier 150 produces an output voltageresponsive to the projection of vector 158 on the axis of vector 122 andthis is again the vector component 124b that is a measure of the unknowncircuit elementconductance G Thus, the multiplier 150 output voltage vis a measure of the conductance G The FIG. 4 demodulator 92 thusproduces three d.c. output voltages that are essentially identical tothe three output voltages from the FIG. 1 demodulator and that can beapplied to suitable output converters or other devices to provide thedesired absolute measures of the capacitance and conductance of thecircuit element 98, references to the capacitance of the referenceelement 102.

Further, as with the FIG. 1 measuring circuit, the FIG. 4 circuit onlyrequires that the modulator 90 be constructed to minimize the strayseries impedances and the strayshunt impedances in the manner discussedabove with reference to FIG. 1. The demodulator 92 can be constructedwithout these restrictions. It only requires that the signal pathdirectly from amplifier 126 to multiplier 148 differ in electricallength from the path from the amplifier to the multiplier by way of thedelay line only by the aforementioned delay of the delay line. Also, theelectrical length of the two signal paths from the amplifier 126 tomultiplier 150, one by way of delay line 152 and the other by way of thephase shifter 154, have the same electrical lengths, exclusive of theprescribed operation of units 152 and 154, as the signal path from theamplifier to the multiplier 148.

Accordingly, the demodulator 92 can be remote from the modulator 90 andconnected to it by a long length of cable 162 between the transformersecondary winding 10417 and the amplifier 126. Alternatively, a wirelesscommunication link can couple the demodulator 92 to the modulator 90 inplace of the cable 162. In either case, in the same manner asillustrated in FIG. 1, this FIG. 4 transmission path between modulator90 and demodulator 92 requires only a single information channel toconvey the (i and i ')-dependent current from transformer 104 toamplifier 126.

The multipliers 148 and 150 in the FIG. 4 demodulator can, by way ofexample, be integrated circuit multipliers. Further, the overallconstruction of this demodulator and of the FIG. 1 demodulator aresimilar to those used in color television receivers.

It should be noted that the demodulator of each measuring circuit, i.e.,in FIG. 1 and in FIG. 4, provides an information storage function, amongother functions. In the FIG. 1 demodulator 34, the sample and holdnetwork of resistor 68 and capacitor 70, with amplifier 72 and theoscillator 74, store information regarding the phase and the frequencyof the reference current. In the demodulator 92 of FIG. 4, on the otherhand, the delay line 152 stores information regarding the referencecurrent phase, frequency and amplitude.

The measuring circuits of both FIG. 1 and of FIG. 4 can be used formeasuring inductive unknown circuit elements equally as well as for thecapacitance measurements described. Where the FIG. 4 circuit element 98being measured has an inductive impedance, rather than the capacitiveimpedance illustrated, the circuit can use the same capacitive referenceelement 102. However, the polarity of the output voltage v which isresponsive to the imaginary component of the impedance of the circuitelement 98, will reverse from the polarity it had for the capacitivecircuit element 98. That is, the polarity of the FIG. 4 voltage v, willchange depending on whether the circuit element 98 is inductive orcapacitive. Also, the FIG. 4 circuit can use an inductive referenceelement in place of the capacitive element 102.

The FIG. 1 measuring circuit also can use the same resistive referenceelement 24 for measuring both inductive and capacitive circuit elements,and the polarity of the v output voltage will change accordingly.

The phase-lock demodulator of FIG. 1 is considered to be more suitablefor use with a modulator having a purely resistive reference element, asillustrated. Likewise the delay line demodulator of FIG. 4 is consideredto be inherently advantageous for use with a modulator having a purelyreactive reference element, again as illustrated.

However, the FIG. 1 measuring circuit can use an inductive or acapacitive reference element, and the FIG. 4 circuit can use a resistivereference element; but in either instance the change will require achange in the corresponding demodulator to account for the differentrelative phase of the resultant signal.

Also, the invention can be practiced with the dual of the illustratedmodulators l and 90. That is, the illustrated modulators apply an r.f.source voltage to the unknown and reference circuit elements, andprocess the resultant currents. A modulator constructed as theelectrical dual applies the current from an r.f. source to the twocircuit elements on a time-sequential basis, and processes the resultantvoltages.

However, whatever form of the measuring circuits one employs willprovide reactance measurement with a minimum of error due to strayreactances. And the modulator portion of the measuring circuit can beconstructed with such small size that it can be placed closely adjacentto a miniature circuit element being measured, as is desired for theaccurate measuring of integrated circuits.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained. Sincecertain changes may be made in the above constructions without departingfrom the scope of the invention, all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention ascribedherein, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

Having described the invention, what is claimed as new and secured byLetters Patent is:

1. Electrical apparatus for producing a measurement of the reactiveelectrical impedance of a first circuit element, said apparatuscomprising A. a pair of terminals for connection to said first circuitelement,

B. a second circuit element of known electrical impedance,

C. current source and switching means connected with said pair ofterminals and with said second circuit element, and producing a singlealternating current with both a phase and a magnitude dependent inalternate time periods first on the reactive impedance between said pairof terminals and second on said known impedance of said second element,said current source and switching means producing said current withrelative phases, in said first and second alternate time periods,dependent substantially only on the reactance of said first circuitelement, and on the reactance of said second circuit element,respectively,

D. signal transmission means providing a single information channel andcoupled in circuit with said current source and switching means, andtransmitting a signal corresponding to said single alternately-dependentcurrent to signal-processing means separated therefrom, and

E. detecting means receiving said signal corresponding to saidalternately-dependent current and receiving a control signal identifyingthe switching of said current source and switching means between saidalternate time periods, and producing in response thereto an outputmeasurement of said reactive impedance between said pair of terminalsrelative to said known impedance.

2. Electrical apparatus as defined in claim 1 in which A. said referencecircuit element has an impedance of known reactance, and

B. said detecting means detects the phase of said alternately-dependentcurrent 'which is dependent on said impedance between said pair ofterminals terminals relative to the phase of said current which isdependent upon said known impedance to produce said output measurement.

3. Electrical apparatus as defined in claim 1 in which said currentsource and switching means comprises A. a source of alternating current,

B. output terminal means at which said alternatelydependent current isdeveloped, and

C. double-pole, double-throw switch means alternately, in successivetime periods, connecting said source current with a first relative phaseof said current in circuit with said pair of terminals and said outputterminal means and, connecting said source current with a reversal ofsaid relative phase of said current in circuit with said second circuitelement and said output terminal means.

4. Electrical apparatus as defined in claim 3 in which said currentsource and switching means further comprises circuit means of lowreactance interconnecting said switch means and output terminal meanswith said pair of terminals and with said second circuit element, anddevelops said current at said output terminal means with phases,relative to the phase of said source, dependent substantiallyexclusively on the reactance of said first circuit element and of saidsecond circuit element, respectively, in each of said alternate timeperiods.

5. A reactance-measuring circuit comprising A. means for modulating atleast the phase of a radio frequency signal in a succession of timeintervals alternately by essentially only the reactance of a firstcircuit element being measured and by essentially only the reactance ofa second reference circuit element,

B. single-channel signal transmission means for delivering saidmodulated radio frequency signal away from said modulating means, and

C. means for receiving said modulated radio frequency signal from saidtransmission means and for detecting at least the phase of themodulation of said signal by said reactance of said first circuitelement relative to the phase of the modulation of said signal by saidknown reactance of said reference circuit element and in timesynchronism with the alternation of said time intervals, and forproviding an output signal responsive to the result of said detection,thereby to provide a measure of the reactance of said first circuitelement.

6; A measuring circuit as defined in claim in which said transmissionmeans consists of a single conductive path interconnecting saidmodulating means and said receiving and detecting means and conductingsaid modulated signal therebetween.

7. A circuit for measuring the impedance of a first circuit elementcomprising A. a radio frequency source of alternating voltage at thefrequency of measurement,

B. a pair of first terminals for connection to said first circuitelement,

- C. a second circuit element of known impedance,

D. switch means having first and second switch conditions and, when insaid first condition connecting said source voltage to said firstterminals to develop therebetween a first current dependent in phase andmagnitude on the impedance of said first circuit element connectedtherebetween, and when in said second condition connecting said sourcevoltage to said second circuit element to develop therein a secondcurrent dependent in phase and magnitude on said known impedance, v

E. switch control means for changing said switch means successivelybetween said first and second conditions,

F. current summing means connected with said first terminals and withsaid second circuit element and developing a further current responsivein successive time intervals to said first current and to said secondcurrent,

G. signal transmission means connected with said summing means andproviding a single information channel for transmitting said furthercurrent to a location removed from said pair of first terminals, and

H. synchronous detector means located removed from said pair of firstterminals and receiving from said transmission means a signalcorresponding to said further current from said summing means andreceiving a control signal corresponding to the condition-changingoperation of said switch control means for producing a first outputsignal responsive to the magnitude of a first component of said firstcurrent having a selected phase relative to said second current.

8. A measuring circuit as defined in claim 7 in which said synchronousdetector means further produces a second output signal responsive to themagnitude of a second component of said first current out of phase withsaid first component thereof.

9. A measuring circuit as defined in claim 7 further characterized inthat said switch means, said current summing means, and circuit meansinterconnecting said switch means and said summing means with said firstterminals and with said second element are substantially free ofreactance so that said first and second currents differ from each otherin phase and magnitude essentially only by the difference between theimpedance of said first circuit element between said first terminals andsaid known impedance of said second circuit element.

10. A measuring circuit as defined in claim 7 in which said synchronousdetector means comprises A. a first detector for producing a referencedirect voltage responsive to a signal corresponding to said secondcurrent from said current means,

B. a phase detector receiving a first input signal corresponding to saidsum of said first and second current from said current summing means,

C. a reference signal source for receiving the output signal from saidphase detector and producing a second input signal to said phasedetector with a frequency and phase that null the phase detector outputsignal, and

D. switch means connected for operation in timed sequence with saidswitch control means for apply.- ing to said first detector only theportion of said first input signal corresponding to said second currentfrom said current summing means, and for applying the phase detectoroutput signal to said reference signal source only during delivery tosaid phase detector of said portion of said first input signalcorresponding to said second current from said current summing means.

11. A measuring circuit as defined in claim 10 further comprising outputconverter means for receiving said reference direct voltage and forreceiving said phase detector output signal responsive to the portion ofsaid first input signal corresponding to said first current from saidcurrent summing means, and producing in response thereto a measure ofthe reactance of said first circuit element relative to said knownimpedance.

l 16 12. A measuring circuit as defined in claim 7 in C. a firstdetector receiving only the portion of said which said synchronousdetector means comprises first input signal corresponding to said secondcur- A. a multiplier receiving a first input signal corrent Output f mSaid summing means and producresponding to said sum of said fi d seconding a reference direct voltage in response thereto. rents from saidcurrent summin m 5 13. A measuring circuit as defined in claim 12 B.delay means for receiving said first input signal to further comprisingOutput converter means receiving said multiplier and applying it to a sed input f said reference direct voltage and the output from said saidmultiplier with a time delay equal to the time multlplier and Producingin response thereto a signal interval during which said switch controlmeans measfmng h reactance of Said first circui element maintains saidswitch means in each of said first 1o relatwe to 531d knownimpedanceandsecond conditions, and

1. Electrical apparatus for producing a measurement of the reactiveelectrical impedance of a first circuit element, said apparatuscomprising A. a pair of terminals for connection to said first circuitelement, B. a second circuit element of known electrical impedance, C.current source and switching means connected with said pair of terminalsand with said second circuit element, and producing a single alternatingcurrent with both a phase and a magnitude dependent in alternate timeperiods first on the reactive impedance between said pair of terminalsand second on said known impedance of said second element, said currentsource and switching means producing said current with relative phases,in said first and second alternate time periods, dependent substantiallyonly on the reactance of said first circuit element, and on thereactance of said second circuit element, respectively, D. signaltransmission means providing a single information channel and coupled incircuit with said current source and switching means, and transmitting asignal corresponding to said single alternately-dependent current tosignal-processing means separated therefrom, and E. detecting meansreceiving said signal corresponding to said alternately-dependentcurrent and receiving a control signal identifying the switching of saidcurrent source and switching means between said alternate time periods,and producing in response thereto an output measurement of said reactiveimpedance between said pair of terminals relative to said knownimpedance.
 2. Electrical apparatus as defined in claim 1 in which A.said reference circuit element has an impedance of known reactance, andB. said detecting means detects the phase of said alternately-dependentcurrent which is dependent on said impedance between said pair ofterminals terminals relative to the phase of said current which isdependent upon said known impedance to produce said output measurement.3. Electrical apparatus as defined in claim 1 in which said currentsource and switching means comprises A. a source of alternating current,B. output terminal means at which said alternately-dependent current isdeveloped, and C. double-pole, double-throw switch means alternately, insuccessive time periods, connecting said source current with a firstrelative phase of said current in circuit with said pair of terminalsand said output terminal means and, connecting said source current witha reversal of said relative phase of said current in circuit with saidsecond circuit element and said output terminal means.
 4. Electricalapparatus as defined in claim 3 in which said current source andswitching means further comprises circuit means of low reactanceinterconnecting said switch means and output terminal means with saidpair of terminals and with said second circuit element, and developssaid current at said output terminal means with phases, relative to thephase of said source, dependent substantially exclusively on thereactance of said first circuit element and of said second circuitelement, respectively, in each of said alternate time periods.
 5. Areactance-measuring circuit comprising A. means for modulating at leastthe phase of a radio frequency signal in a succession of time intervalsalternately by essentially only the reactance of a first circuit elementbeing measured and by essentially only the reactance of a secondreference circuit element, B. single-channel signal transmission meansfor delivering said modulated radio frequency signal away from saidmodulating means, and C. means for receiving said modulated radiofrequency signal from said Transmission means and for detecting at leastthe phase of the modulation of said signal by said reactance of saidfirst circuit element relative to the phase of the modulation of saidsignal by said known reactance of said reference circuit element and intime synchronism with the alternation of said time intervals, and forproviding an output signal responsive to the result of said detection,thereby to provide a measure of the reactance of said first circuitelement.
 6. A measuring circuit as defined in claim 5 in which saidtransmission means consists of a single conductive path interconnectingsaid modulating means and said receiving and detecting means andconducting said modulated signal therebetween.
 7. A circuit formeasuring the impedance of a first circuit element comprising A. a radiofrequency source of alternating voltage at the frequency of measurement,B. a pair of first terminals for connection to said first circuitelement, C. a second circuit element of known impedance, D. switch meanshaving first and second switch conditions and, when in said firstcondition connecting said source voltage to said first terminals todevelop therebetween a first current dependent in phase and magnitude onthe impedance of said first circuit element connected therebetween, andwhen in said second condition connecting said source voltage to saidsecond circuit element to develop therein a second current dependent inphase and magnitude on said known impedance, E. switch control means forchanging said switch means successively between said first and secondconditions, F. current summing means connected with said first terminalsand with said second circuit element and developing a further currentresponsive in successive time intervals to said first current and tosaid second current, G. signal transmission means connected with saidsumming means and providing a single information channel fortransmitting said further current to a location removed from said pairof first terminals, and H. synchronous detector means located removedfrom said pair of first terminals and receiving from said transmissionmeans a signal corresponding to said further current from said summingmeans and receiving a control signal corresponding to thecondition-changing operation of said switch control means for producinga first output signal responsive to the magnitude of a first componentof said first current having a selected phase relative to said secondcurrent.
 8. A measuring circuit as defined in claim 7 in which saidsynchronous detector means further produces a second output signalresponsive to the magnitude of a second component of said first current90* out of phase with said first component thereof.
 9. A measuringcircuit as defined in claim 7 further characterized in that said switchmeans, said current summing means, and circuit means interconnectingsaid switch means and said summing means with said first terminals andwith said second element are substantially free of reactance so thatsaid first and second currents differ from each other in phase andmagnitude essentially only by the difference between the impedance ofsaid first circuit element between said first terminals and said knownimpedance of said second circuit element.
 10. A measuring circuit asdefined in claim 7 in which said synchronous detector means comprises A.a first detector for producing a reference direct voltage responsive toa signal corresponding to said second current from said current means,B. a phase detector receiving a first input signal corresponding to saidsum of said first and second current from said current summing means, C.a reference signal source for receiving the output signal from saidphase detector and producing a second input signal to said phasedetector with a frequency and phase that null the phase detector outputsignal, and D. switch means connected for operation in timed sequencewith said switch control means for applying to said first detector onlythe portion of said first input signal corresponding to said secondcurrent from said current summing means, and for applying the phasedetector output signal to said reference signal source only duringdelivery to said phase detector of said portion of said first inputsignal corresponding to said second current from said current summingmeans.
 11. A measuring circuit as defined in claim 10 further comprisingoutput converter means for receiving said reference direct voltage andfor receiving said phase detector output signal responsive to theportion of said first input signal corresponding to said first currentfrom said current summing means, and producing in response thereto ameasure of the reactance of said first circuit element relative to saidknown impedance.
 12. A measuring circuit as defined in claim 7 in whichsaid synchronous detector means comprises A. a multiplier receiving afirst input signal corresponding to said sum of said first and secondcurrents from said current summing means, B. delay means for receivingsaid first input signal to said multiplier and applying it to a secondinput of said multiplier with a time delay equal to the time intervalduring which said switch control means maintains said switch means ineach of said first and second conditions, and C. a first detectorreceiving only the portion of said first input signal corresponding tosaid second current output from said summing means and producing areference direct voltage in response thereto.
 13. A measuring circuit asdefined in claim 12 further comprising output converter means receivingsaid reference direct voltage and the output from said multiplier, andproducing in response thereto a signal measuring the reactance of saidfirst circuit element relative to said known impedance.